CRT is a vacuum tube device where an
electron beam can be focused onto a small spot on the fluorescent screen
at the opposite end of the tube. The screen is coated with a matrix of
thousands of tiny phosphor dots, which glow when it is struck by a stream
of fast moving electrons. Different phosphors emit different coloured
light and each dot consists of three blobs of coloured phosphor: red,
green and blue which will make up as a single
pixel.

Figure 1: Components of CRT

The electron gun in the “bottle neck”
of the CRT is composed of a cathode, heat source and focusing elements.
There are three different electron guns in a colour monitor, one for each
phosphor colour. Images are created when high speed electrons emitted from
the heated cathode, converged to strike their respective phosphor blobs.
Precise
convergence is important as
CRT display works on the principal of additive coloration, whereby
combinations of different intensities of red, blue and green phosphors
create the illusion of millions of colour. When each of the primary colours
is added in equal amounts, white spot is formed, whereas the absence of any
colour will result a black spot. Misconvergence will show up as shadows
which appear around text and graphic images.

The electron gun emits
electrons when the cathode (negatively charged) is heated by the heater to
liberate electrons. In order for the electron to reach the phosphor screen,
it has to pass through the focusing element. While the emitted electron beam
will be circular in the middle of the screen, it has a tendency to become
elliptical as it spreads its outer areas, creating a distorted image. The
focusing elements are set up in a way so that the electron flow is focused
into a very thin beam in a specific direction. Then the positively charged
anode (located near the screen) will draw the electrons toward the phosphor
dots, and cause the electron beam to light up a specific dot.

Figure 2: Electron gun

The deflection yoke around
the CRT’s neck creates a magnetic field that directs the electron beams to
strike the appropriate position on the screen. This starts at the top left
corner (as viewed from front) and flashes on and off as it moves across the
row or “raster”, from left to right.
As it reaches the edge of the screen, it stops and moves down to the next
line. Its motion from right to left is called horizontal retrace and is
timed to match with the horizontal blanking interval so that the retrace
lines will be invisible. This process is repeated until every line on the
screen is traced, at which point it moves from the bottom to the top of the
screen – during the vertical retrace interval – ready to display the next
screen image. During this process, the electron guns are controlled by the
video data stream coming into the monitor from the video card, which varies
the intensity of the electron beam at each position on the screen.

The control of the intensity
of the electron beam at each dot controls the colour and brightness of each
pixel on the screen and it happens very quickly, and in fact the entire
screen is drawn in a small fraction of a second. There are separate video
streams for each colour coming from the video card, which allows the
different colours to have different intensities at each point on the screen.
The full rainbow colour is made possible, by varying the intensity of the
red, green and blue streams. The surface of the CRT only glows for a small
fraction of a second before beginning to fade. Thus the monitor must redraw
the picture many times per second to avoid having the screen flicker as it
begins to fade and then is renewed. This rapid redrawing is called
“refreshing” the screen.

The dot pitch of a monitor
is the physical distance between adjacent phosphor dots of the same colour
on the inner surface of the CRT. Typically, this is between 0.22mm and
0.3mm. The smaller number gives a finer and better resolved image. However,
providing too many pixels to a monitor without sufficient dot pitch to cope
causes very fine details, like the writing below icons to appear blurred.

The electron beam travels
through a perforated sheet located in front of the phosphor before it
strikes the phosphor dots. Originally known as “shadow mask”, these sheets
are now available in a number of forms, designed to suit the various CRT
tube technologies that have emerged over the years. They have a number of
important functions, namely:

“mask” the electron
beam, forming a smaller, more rounded point that can strike individual
phosphor dots cleanly

Filter out stray
electrons, thereby minimizing “overspill” and ensuring that only the
intended phosphors are hit

Guide
the electrons to the correct phosphor colours, permit independent control
of brightness of the monitor’s three primary colours.

Another alternative is by
using an aperture grill where hundreds of fine metal strips run vertically
from the top of the screen surface to the bottom. Like the shadow mask, it
also forces the electron beam to light up only the correct parts of the
screen. The advantage of using aperture grill is that it allows more than
one electron beam to pass through the phosphor resulting in a brighter
overall picture. Finally, as these strips are aligned vertically from top to
bottom of a monitor, this type of tube is flat vertically; it curves outward
as you go from left to middle to right, but not as you go from top to
middle to bottom of the monitor. This has reduced glare and results in a
more pleasant and less distorted images. However, the major disadvantage is
that this bunch of thin metal strips does not have the same physical
stability like the shadow mask and tend to vibrate. Hence to correct this
problem, one, two or three thin stabilizing wires are aligned horizontally
across the screen. Although it eliminates problems with the metal strips
moving around, it cause the appearance of very faint lines where the
stabilizing wires are.

Initiative has
been taken to reduce power consumption of monitor during idle periods
because of the large amount of energy consumed by monitors during its
operations. Most modern monitors are compliant with
VESA’s DPMS protocol. It is used
selectively to shut down parts of the monitor circuitry after a period of
inactivity.

Phosphor burn in and screen saver

If a
particular image is displayed on a screen for a long time, the same dots
will be strike by the electron beam repeatedly millions of time and cause
the surface of the CRT to be damaged after some time. As this happen,
‘ghosting’ can be seen on the surface of the screen and the outline of the
image that was displayed so many times could even be seen when the CRT is
turned off. When this happens the phosphor is ‘burnt in’. Thus, a screen
saver has been created to prevent burn in of the screen phosphor. A screen
saver is just a software program that blanks the screen or display moving
pattern on it, after a specified period of inactivity.

Electronic emission

The electron
beam that creates the image produces electrical and magnetic fields as a
side-effect. However, it is unknown of to what extent does these emissions
can be linked to health problems. Some believed that prolonged exposure to
these electromagnetic fields can lead to increased risk of cancer.

Keep the cover on for safety’s sake

Monitors
operate on a very high voltage and have special hazards that can cause
serious injury or even death, if you make a mistake while working on one.
This is true even when the power of your monitor is turned off, due to the
large capacitor that holds charges inside the CRT. Thus, it is dangerous to
tamper with devices containing CRT tubes unless you have an engineering
training and have taken appropriate pre-cautions.